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Proceedings of the International workshop on Arsenic in Food Chain: Cause, Effect and Mitigation, 20th February, 2012, Kolkata, India, DNGM Research Foundation, Kolkata, p.36-46, 2012

Review on arsenic contamination in Inland open water ecosystems

(Arsenic and Water Ecosystem)

Atalanta Narayan Chowdhury, Srikanta Samanta

Abstract

The paper reviews the occurrence and distribution of arsenic (As) in the inland aquatic environments. Reported typical water As concentrations in the freshwater ecosystems is less than 10 µg l-1 and often less than 1 µg l-1 but for contaminated water bodies the toxicant has been found even up to thousands of microgram per litre. The world average sediment arsenic content is usually less than 10 mg kg−1 and for river sediments the baseline level is 5 mg kg-1. A significant proportion of water As is contributed by the sediment phase and the amount of arsenic release from sediments is governed by its physico-chemical properties and biological activities. Human exposure of arsenic through food chain is now well established and since fish is one of the major dietary components, it has been widely studied. For various fresh water species the reported range is from traces to as high as 22 µg g-1. As a case study, As status in the abiotic and biotic components of inland water ecosystems from affected areas of Nadia District in comparison to the unaffected area from Hooghly District, West Bengal has been presented. The study emphasized the beneficial role of using the surface water bodies over the highly contaminated ground water for various livelihood activities. Fish consumption is also encouraged for combating arsenic toxicity as the observed As levels are relatively less.

Keywords: Arsenic contamination, inland water ecosystem, water, sediment, fish

Introduction

Arsenic (As, atomic number= 33; relative atomic mass= 74.92), being the component of more than 245 minerals, occurs naturally in the earth's crust. These minerals are mostly ores containing sulfide of copper, nickel, lead, cobalt or other metals. Arsenic ranks 20 th in natural abundance, 14th in seawater, and 12th in the human body [1]. Since its isolation in 1250 A.D., it has been used in various fields such as medicine, agriculture, electronics, livestock, and metallurgy [2]. Apart from natural processes like volcanic eruptions and weathering of arsenic bearing minerals, elevated concentrations of the metalloid in natural waters are usually associated with anthropogenic activities like disposal of industrial waste, mining, smelting, burning of fossil fuels, the application of arsenic compounds in many products, agricultural use and irrigation practices [3, 4].

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Arsenic is listed by the Environmental Protection Agency as one of the priority pollutants. Arsenic is also listed among the most hazardous substances thought to pose the most significant potential threat to human health [5]. Of the various sources of As, drinking water is the major problem creator to human health. Drinking water is derived from a variety of sources depending on local availability: surface water (rivers, lakes, reservoirs and ponds), groundwater (aquifers) and rain water. The elevated level in groundwater appears as point sources of As contamination. Interestingly, storing of contaminated groundwater for agriculture and aquaculture practices; disposal of As bearing domestic and industrial (timber, tannery, paints, electroplating etc.) waste; input of agricultural run-off from the crop lands of arsenic affected areas to the surface water bodies lead to overall ecosystem perturbations. A schematic diagram of Arsenic dynamics in nature is presented in Fig. 1.Unlike groundwater, the arsenic contamination issue in the surface-water ecosystems has not been well addressed till now. Although the arsenic associated problems are acute in various part of the world, only meager information is available regarding quantification and distribution of the element in different components of open-water ecosystems. The purpose of this review paper is to summarize the available literatures on arsenic contamination in the inland surface water ecosystems.

Figure 1: Arsenic dynamics in contaminated soil and aquatic ecosystems (reproduced from Mahimairaja et al., 2005)

Arsenic in natural waters

The widespread occurrence of high concentrations of As in water in many parts of the world caused the U.S. President George W. Bush to state “Arsenic is a natural substance that sometimes causes problems,” and to reverse the previous government’s decision to accept a five times lower WHO standard (i.e., 10 µg liter-1) [6].

Occurrence

Both inorganic and organic forms of arsenic have been determined in water [7]. The dissolved forms in the water column include arsenate, arsenite, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) [8]. The bioavailability, mobility and toxicity of the metalloid are determined by its oxidation state, which in turn depends on the biotic and abiotic conditions of the ecosphere.

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Generally some of the species have an affinity for clay mineral surfaces and organic matter, and this can affect their environmental behaviour. The solubility of arsenic species in water and reaction involving interchange of their oxidation states is usually controlled by pH, redox conditions, biological activity and adsorption reactions [9]. Transport and partitioning of arsenic in water depends on the chemical form of the arsenic and on interactions with other materials present. Methylation and demethylation are also important transformation reactions controlling the mobilization and subsequent distribution of arsenicals [10]. The predominant arsenic species in the water column of surface water bodies is arsenate, as expected in oxidizing environments [11]. Arsenite is usually present and sometimes dominates in bottom water which contains high concentrations of Fe(II) and low oxygen. Peterson & Carpenter [12] reported that the arsenate: arsenite concentration ratio was 15 : 1 in the oxic region of the water column and 1 : 12 in the anoxic zone. Anaerobic conditions led to aqueous arsenic levels, principally as arsenite, about 10 times higher than concentrations reached with aerobic conditions [13]. Seasonal trends reveal higher concentrations of arsenic in summer than in winter. A significant proportion of As in aquatic environment is derived from the sediments, and the relative distribution of As in water and sediments depends mainly on the nature and amounts of sediments. The distribution and transport of arsenic in sediment is a complex process that depends on water quality, native biota and sediment type. There is a potential for arsenic release when there is fluctuation in Eh, pH, soluble arsenic concentration and sediment organic content [14]. Arsenic may be adsorbed from water on to clays, iron oxides, aluminium hydroxides, manganese compounds and organic material [15, 16]. Arsenate and arsenite differ in adsorption characteristics, and this influences their mobilization and subsequent distribution during water–sediment interactions. Reducing conditions increases the release of arsenic from sediments to surface waters as a result of the release of arsenite from iron oxyhydroxide phases, but oxidizing conditions could reduce the mobility because of enhanced adsorption of arsenate species. However, a large reduction in pH (to ≤4) would enhance the mobility of arsenic even under oxidizing conditions. Thus the constituents and environmental conditions of the sediment have a greater influence on arsenic speciation and mobility than the total concentration. Clement & Faust [13] found that adsorption–desorption equilibria and the amount of ‘available’ arsenic present in the sediment greatly influenced the soluble arsenic concentration found in the aqueous phase. Oxides immobilize arsenite and arsenate by adsorption. Organic content, soil fractions and oxides of Al, Fe and Mn also play a vital role. Sediments with finer texture usually contain more arsenic than with coarser texture. Microbial reduction and methylation to arsine solubilize the arsenic, and diffusion through the sediments or mixing by currents or burrowing organisms cause arsenic to re-enter the water column. Natural geological sources of As to drinking water are one of the most significant causes of arsenic contamination around the world. According to the U.S Environmental Protection Agency (2002) [17] and WHO (2004) [18], the maximum contaminant level (MCL) of arsenic in drinking water, a highest permissible arsenic level is 10 µg l-1. However the guideline in many developing countries, including India & Bangladesh, is still based on an earlier advice of 50 µg l-1. Contamination of groundwater aquifer and occurrence of arsenic above permissible level in drinking water is the most accepted mechanism of widespread arsenic poisoning.

Occurrence in Inland Open Water Ecosystems

Water

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Arsenic has been found in many natural surface water bodies like rivers, lakes, wetlands, ponds etc. The average aqueous As concentrations is generally less than 10 μg l−1 [19]. Sharma and Sohn [20] proposed a level of <0.001 µg l-1 for uncontaminated freshwater where as Bissen and Frimmel [3] reported the range of 0.15 – 0.45 μg l−1 for the same. The levels of arsenic in surface water may be upto 100 – 5000 μg l−1 in areas of sulfide mineralization and mining [21]. Most arsenic in natural water is a mixture of arsenate and arsenite, with arsenate usually predominating. In seawater, the concentration of arsenic is usually less than 2 μg l−1 [22].Baseline concentrations of As in river waters were low with an average value of 0.8 μg l−1 [range: 0.13 – 2.1 μg l−1] [23, 24]. They vary according to the composition of the surface recharge, the contribution from baseflow and the bedrock lithology. Low average concentrations of about 0.25 μg l−1 (range: <0.02–1.1) and 0.15 – 0.45 μg l−1 in rivers from Norway and south-eastern USA has been found [25, 26]. Sonderegger and Ohguchi [27] reported a value of 2.1 μg l−1 for USA river water. Seyler and Martin [28] found average river concentrations as low as 0.13 μg l−1 in the Krka region of Yugoslavia. For uncontaminated River Danube, Bavaria; River Dordogne, France; River Po, Italy; and Schelde catchment, Belgium the reported average water As levels were 3 (range: 1 – 8) μg l−1

[29]; 1.3 μg l−1 [30]; 0.7 μg l−1 [31]; and 0.75 – 3.8 (up to 30) μg l−1 respectively [32]. In three non polluted river Nile coastal lakes in Egypt As levels lie within the range 1.2 to 18.2 μg l−1 [33]. Arsenic levels in Canadian surface waters away from point sources of contaminants are typically < 2 μg l−1 [34]. Relatively high concentrations of naturally-occurring As can occur in some areas as a result of inputs from geothermal sources or high-As groundwaters. Arsenic concentrations in river waters from geothermal areas have been reported upto 8.5 mg l−1 and 1.8 – 6.4 mg l−1 in New Zealand and Japan, respectively [35, 36]. Aggett and Aspell [37] reported a level of 37 – 60 μg l−1 for Lake Ohakuri from geothermal release areas of New Zealand. A level of 3 – 121 μg l−1 for river Waikato was reported [38]. As concentrations up to 370 μg l−1 in Madison and Missouri River water (Wyoming and Montana) was found as a result of geothermal inputs from the Yellowstone geothermal system [39]. Wilkie and Hering [40] also found concentrations in the range 85 – 153 μg l−1 in Hot Creek, tributary of the Owens River, California.Concentrations of naturally-occurring As in surface waters from the Loa River Basin of northern Chile, ranging between 190 and 21,800 μg l−1 [41] which was well correlated with salinity whereas Sancha [42] reported a range of 400–450 μg l−1 from same area. Elevated level of 950 – 13,080 μg l−1

for natural water from Chile and the source of contamination was geological in nature was found [43]. Increased As concentrations (up to 114 μg l−1) have also been reported in river waters from central Argentina where regional groundwater As concentrations are high[44]. Arsenic concentrations in river water samples from Bangladesh have been reported in the range < 0.5 – 2.7 μg l−1 but with one sample having a high concentration of 29 μg l−1 [45]. Interestingly, very high concentration of arsenic (up to 300 µg l-1) was also recorded in Shivnath River from central-east India [46] and so far, this is the highest recorded concentration reported from the open water resources of India.Elevated arsenic concentration of surface water also caused from mine wastes and mill tailings. Some researcher found concentrations up to 556 μg l−1 (average 17.5 μg l−1) in streams adjacent to tailings deposits in British Columbia, Canada [47]. A study by Hunt and Howard [48] from England (SW) reported a dissolved As(III) level of 240 μg l−1 for river water receiving tin mine drainage. The

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report from the downstream of Moira River, Ontario, revealed an elevated As concentration up to 23 μg l−1 [49] due to influence of tailings from gold-mine workings. Surface water As level of 218 (range: 4.8 – 583) μg l−1 was reported from Thailand [50] and others noted high As concentrations in surface waters of Ghana 284 (<2 – 7900) μg l−1 affected respectively by Sn- and Au- mining activities [21]. Elevated concentrations of arsenic have been reported in surface waters in the vicinity of gold-mining or ore roasting operations. Mean levels of about 45 μg l−1 with a maximum of 140 μg l−1 were found near abandoned gold mines in Nova Scotia and Ontario [5].Significant increases in aqueous As concentrations may also occur as a result of pollution from industrial or sewage effluents. One of such example was the River Zenne from Belgium. Andreae and Andreae [32] found water As concentrations up to 30 μg l−1. However, the concentration of As in water from most of the catchment was in the range 0.75 – 3.8 μg l−1 and not significantly different from baseline concentrations. Another example was the Manchar lake of Pakisthan. A increased As level of 35 – 157 µg l-1 was reported for the water body [51].Concentrations of As in lake waters are typically close to or lower than those found in river water. Baseline concentrations have been found at <1 μg l−1 from various part of the world. several researcher found baseline lake water As concentrations in the range 0.73 – 9.2, 0.38 – 1.9 and 0.06 – 1.2 μg l−1 from France, Japan and Sweden respectively [11,52, 53]. Azcue and Nriagu [49] reported an average water As level of 0.28 (range: <0.2–0.42) μg l−1 for British Columbia and 0.7 μg l−1 for Ontario. Increased As concentrations are found in lake waters affected by geothermal water and by mining activity. Ranges of typically 100 – 500 have been reported in some mining areas and up to 1000 μg l−1 in geothermal areas [54].From the central-east India it was reported that total arsenic content of pond water 62 ± 52 µg l -1

(range: 15 - 221 µg l-1) [55]. Sanyal and Dhillon [56] report the range 4 – 70 µg l-1 from limited numbers of ponds from contaminated villages of Nadia District, West Bengal, India. It was reported from the adjoining Jessore district of Bangladesh the level of 14 - 54 µg l -1[57]. Some other literatures suggest the level of 30 – 103 µg l-1 from the ponds of south-west Taiwan [58].

Sediment

Sediments in aquatic systems mostly possessed higher arsenic concentrations than those of the water. The natural level of arsenic in sediments is usually below 10 mg kg−1 [59] and varies considerably all over the world. Fine-grained sediment would typically contain more arsenic than course-grained sediment, and sediments higher in total organic carbon (TOC) would typically have more arsenic than sediments which are similar except for being lower in TOC. Muds and clays usually have higher concentrations than sands and carbonates. Elevated concentrations of the toxicant favoured by abundance of sulphide minerals (pyrite) or Fe oxides. Sediments having the contamination <3 mg kg-

1; 3 - 8 mg kg-1 and > 8 mg kg-1 are considered as non-polluted, moderately polluted and heavily polluted respectively [60, 61]. Martin and Whitfield [62] reported 5 mg kg-1 as world average for As in river sediments. Azcue and Nriagu [49], Smedley & Kinniburgh [54] and Bissen and Frimmel [3] reported average sediments arsenic contents in the range of 3 (range: 0.6 – 50) mg kg-1, 3 to 10 mg kg-1 and 0.1 - 490 mg kg-1. Most sediment arsenic concentrations reported for U S rivers, lakes, and streams range from 0.1 to 4,000 mg kg-1, but much higher levels may occur in areas of contamination [19]. Various workers

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noticed highly variable arsenic concentration in river sediments, ranging from 32.8 – 42.7 mg kg -1

(Australia) [63] to 8700 – 156100 mg kg-1 (New Zealand) [38]. A average As concentration of 347 mg kg-1 for river sediment of Brazil was found [64]. Datta and Subramanian [65] found concentrations in sediments from the river Ganges averaging 2.0 (range: 1.2 – 2.6) mg kg-1, from the Brahmaputra river averaging 2.8 (range 1.4 – 5.9) mg kg-1 and from the Meghna river averaging 3.5 (range 1.3 – 5.6) mg kg-1. An average sediments As content of 22.1 (range: 9.0–28) mg kg-1 from Bangladesh [66]. A study from Hanoi, Vietnam a level of 1 – 3050 mg kg-1 for arsenic-rich sediment samples [67]. Eisler [68] reported arsenic concentrations in sediments from areas contaminated by arsenical herbicides had ranging from 198 to 3500 mg kg-1. As level of Lake sediment of Lake Superior and British Colombia had been reported 2.0 (range: 0.5 – 8.0) mg kg-1 [69] and 5.5 (0.9 – 44) mg kg-1 [70] respectively. Average As concentrations for stream sediments in England and Wales are in the range 5 – 8 mg kg-1 [71]. Sediment arsenic concentration of 11 – 56 mg kg-1 has been reported [72] from contaminated Manchar Lake of Pakisthan with a mean of 27 mg kg-1. Sediment phase of the Chattishgarh state (central-east India) was reported to be highly contaminated with arsenic (range: 10.7 – 200.0; 68.0 ± 8.4 mg kg-1) [46]. In some of the studied ponds of this area arsenic content was found up to 167 mg kg-1 [46] or even more 19 - 489 mg kg-1 (105 ± 130 mg kg-1) [46], reason for the contamination appears to be geologic. The As-rich sediments act as a buffer in maintaining the As concentration in water bodies, thereby controlling the dynamics and bioavailability of As in the aquatic environment.

Fish

Fish, most important aquatic organism, serves as one of the major protein source in diet for a large part of world population. Various types of fishes have been studied throughout the world for their arsenic contamination level and contribution in arsenic exposure through food chain. As early as late 80’s it was reported that arsenic concentrations of 0.26 – 0.40 μg g-1 (fresh weight) for inland water fishes of African continent [73, 74] where as report of a mean value of 0.031 μg g -1 for finfish from Lakes ldku, Egypt [75]. For various fresh water fishes collected in 1984 - 1985 from 109 stations throughout USA, Schmitt and Brumbaugh [76] reported a maximum detected level of 1.5 μg g-1 (wet weight) [with a mean of 0.14 μg g-1] for whole fish. Azcue and Dixon [77] reported a level of 0.025 – 2.36 μg g-1 (wet weight) for various samples collected from Moira Lake, Canada. At the same time Baker and King [78] reported a level of 0.100 – 0.200 μg g-1 (wet weight) for Catfish & Carp of Gila River, USA. Mean total arsenic concentration of muscle tissue of fresh water fish species of Back Bay, Canada were reported in the range of 0.28 - 3.10 μg g-1 by Koch [79] and later 0.57 - 1.15 μg g-1

(dry weight) by other authors [80]. During last decade the accumulation of arsenic in fishes has been studied extensively by researchers throughout the world. Mean arsenic levels of 0.051 – 0.370 μg g-1, 0.015 – 0.036 μg g-1, 0.019 – 0.136 μg g-1 and 0.1-0.3 μg g-1 (all in wet weight) for muscle tissue of various freshwater fishes from Hayakawa River, Japan; Mystic Lake, USA; various surface water bodies and Harrington Creek, USA was observed respectively [81- 84]. Cooper and Gillespie [85] noticed a value of 0.026 μg g-1 (wet weight) for omnivores of Moon Lake (USA). From Jordon market sample a mean value of 1.39 ± 0.11 μg g-1 in Acanthobrama, a freshwater species imported from Yemen was reported [86]. It was reported that arsenic values ranged from 0.03 μg g-1 to 0.32 μg g-1 (wet weight) in muscles of several species of fish from the Savannah River of Southeastern

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United States [87]. Fish from Turia River (Spain), S. trutta accumulates 0.06 μg g-1 with a maximum value of 0.12 μg g-1 (wet weight) of arsenic in muscle [88]. Odonthestes microleptus from a moderately arsenic-impacted river of Argentina contained 0.25 μg g-1 of arsenic in muscle tissue [89]. In arsenic-rich Slovenian rivers arsenic values in muscles ranged from 0.08 to 1.235 μg g-1 (wet weight) for several salmonid species [90]. In the Tiber River (Italy), it was reported that arsenic values ranged from 0.73 μg g-1 to 0.939 μg g-1 (dry weight) in muscles of Anguilla anguilla and from 1.14 μg g-1 to 3.62 μg g-1 (dry weight) in the muscle of Leuciscus cephalus [91]. It was found that carp (Cyprinus carpio) cultured in Hungarian lakes with low arsenic contamination (0.7 to 13.2 μg l -

1) had concentrations of total arsenic ranging from 0.062 to 0.363 μg g-1 (dry weight) in muscle [92]. Alinnor [93] reported a level of traces to 0.01 μg g-1 for whole fish from Aba River (Nigeria). It was also reported that a level of 0.02–0.26 μg g-1 for fishes of River Neretva (Croatia) [94]. Jankong reported [95] mean arsenic concentrations in muscle tissue of Channa striata, Danio regina and Rasbora heteromorpha from a highly contaminated pond in Thailand were, respectively, 22.2 μg g-1, 8.8 μg g-1 and 7.9 μg g-1 (dry weight).Zhang found [96] the mean concentrations of 0.113 μg g-1

with a range of 0 - 0.27 μg g-1 in muscle of freshwater fishes collected from the Banan section in the Three Gorges reservoir, Yangtze River, China. Cheung et al. (2008) [97] reported a range of 0.53 (± 0.10) - 2.11 (± 0.16) μg g-1 (dry weight) for Bighead carp; 0.42 (± 0.52) - 2.24 (± 0.27) μg g-1 (dry weight) for Tilapia; 0.73 (± 0.07) - 3.44 (± 0.40) μg g-1 (dry weight) for Grass Carp and 1.80 (± 0.33) - 2.73 (± 0.37) μg g-1 (dry weight) for Mandarin collected from local fish ponds and the level was 0.24 - 2.13 μg g-1 (dry weight) for fishes collected from Local market of China. In fish muscle from the non-contaminated Svitana Lake (Bosnia and Herzegovinia), [94] found arsenic levels (wet weight) in the range of 0.007 – 0.161 μg g-1 and Hasselquist (mean: 0.152 ± 0.007, range: 0.139–0.161 μg g-1) was found to be mostly contaminated species. Culioli reported [98] in the muscle tissue arsenic levels of S. trutta from the Presa River and Bravona River (France) were in the range of 0.88 to 2.49 μg g-1 (dry weight; mean: 1.45 ± 0.51 μg g-1) and upto 0.88 μg g-1 respectively. Arsenic in different fish tissues were analyzed and showed significant higher level in fish muscle (2.12 – 15.2 μg g−1) from Sindh, Pakisthan [99]. The highest level of As was observed in muscles of Catla Catla (15.2 μg g−1) from this zone. Raissy reported [100] arsenic concentration of 35 - 88 μg kg-1 (mean 57.5 μg kg-1) for fishes from Beheshtabad River of Iran. Kar reported [101] the mean arsenic levels in tissue of tilapia in the range from 0.43 (± 0.01) - 1.91 (± 0.05) μg g−1.

Guideline Values of Arsenic for Aquatic Ecosystems

The goal of the water and sediment quality guidelines (Table 1 and 2) is the protection and maintenance of all forms of aquatic life and their stages in the freshwater environment vis-a-vis management of these ecosystems.“The Criterion Continuous Concentration (CCC) is an estimate of the highest concentration of a material in surface water to which an aquatic community can be exposed indefinitely without resulting in an unacceptable effect. The Criteria Maximum Concentration (CMC) is an estimate of the highest concentration of a material in surface water to which an aquatic community can be exposed briefly without resulting in an unacceptable effect.” (US EPA, 2009). The recommended water quality criterion was derived from data for arsenic (III), but is applied here to total arsenic, which might imply that arsenic (III) and arsenic (V) are equally toxix to aquatic life and tht their toxicities are additive.

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The Lowest Effect Level (LEL) indicates clean to marginally polluted sediment quality, which can be tolerated by most benthic species. The Severe Effect Level (SEL) indicates heavily polluted sediment that is likely to affect the health of most benthic animals and may be acutely toxic. From a broad survey of literature values based on field and laboratory studies, the USEPA [102] derived a Consensus-Based Probable Effect Concentration (PEC), that were established as concentrations of individual toxicant above which adverse effects are expected to frequently occur. Canadian Interim Sediment Quality Guidelines (ISQG) and Probable Effect Levels (PEL) for As can be used to evaluate the degree to which adverse biological effects are likely to occur as a result of exposure to total As in surficial sediments (i.e., top 5 cm). These levels for As were developed using a modification of the National Status and Trends Progrrame approach as described in CCME [103].

Table 2: Sediment quality guidelines for protection of aquatic life

Different benchmarks for fresh water bodies Level of As (mg kg-1 on dry weight)

Reference

Lowest effect level (LEL) 6.0 Persaud et al. 1993Severe effect level (SEL) 33.0 Persaud et al. 1993Interim sediment quality guidelines (ISQG) 5.9 CCME 2002Probable effect level (PEL) 17.0 CCME 2002Consensus-based probable effect concentration (PEC) 33.0 Ingersoll et al., 2000

Arsenic Contamination in pond ecosystems from affected areas of rural Bengal: a case studyStudy has been performed on pond ecosystems of arsenic affected areas from Chakdaha and Haringhata block of Nadia district and compared with uncontaminated areas of Polba block of Hooghly District of West Bengal. Following levels of the toxicant were observed (Table 3).

Table 3: Arsenic level in pond water and sediment samples from affected and unaffected area (unpublished data)

Site N Range Mean ± SE

Water Arsenic Content (µg l-1) Contaminated Villages 277 2 – 174 31 ± 1.6Uncontaminated Village 90 1 – 8 4 ± 0.3

Sediment Arsenic Content (mg kg-1) Contaminated Villages 277 1.34 – 37.31 10.27 ± 0.4Uncontaminated Village 90 1.42 – 5.24 2.96 ± 0.1

The pond water and sediment arsenic concentration of contaminated villages varied widely. Statistical analysis clearly differentiated the pond water and sediment arsenic content of control village from affected villages. Our study revealed that ponds receiving ground water and agricultural effluents had significantly high arsenic accumulation and seasonal variations were found statistically

Table 1: Water quality guidelines for protection of aquatic lifeDifferent benchmarks for fresh water bodies Level of Arsenic (µg l-1) ReferenceGuideline value 5.0 CCME 2001Criterion continuous concentration (CCC) 150.0 US EPA 2009Criteria maximum concentration (CMC) 340.0 US EPA 2009

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insignificant. A moderately strong correlation was observed for the water and sediment arsenic level from contaminated region (r = 0.688, n = 277, at 0.01% level of significance) but such correlation was poor for the ponds from uncontaminated zone (r = 0.149, n = 90, at 32.73% level of insignificance).The average water arsenic contents of all the studied ponds were found higher than 5 µg l-1, the guideline value for the protection of aquatic life by CCME [104]. More than 95% of analyzed pond water samples from affected villages showed As concentration higher than 5 µg l-1. Only 3 samples were found having As higher than Criterion continuous concentration (CCC) level but none with As content higher than Criteria maximum concentration (CMC) level of USEPA [105]. Thus, observed water arsenic level of contaminated ponds were found safe for aquatic lives with respect to the USEPA guideline, but the CCME guideline indicated that the studied ecosystems are detrimental for the aquatic community. All the ponds from reference area had water As concentration well below 5 µg l-1 and were found safe.From the presently studied affected area, 79% of the analysed sediment samples had arsenic status higher than Lowest effect level (LEL) and Interim sediment quality guidelines (ISQG); about 12% showed the accumulation higher than Probable effect level (PEL) and only in 4 samples the toxicant was found above Severe effect level (SEL) or Consensus-based probable effect concentration (PEC). On comparing the mean sediment arsenic content with the guideline of New York State Department of Environmental Conservation (1999; As > 6.0 mg kg-1 - moderately contaminated & As > 33.0 mg kg-1 - severely contaminated), 69% of studied ponds were found to be moderately contaminated and none severely affected. The above discussion suggests that the affected ponds are still useful for aquaculture/ fish production but due precautions should be taken in the management practices. All ponds from the unaffected area had arsenic well below LEL/ ISQG and safe.We have also studied the aquatic biota for the arsenic contamination status and found the following level for the metalloid (Table 4).

Table 4: Arsenic level in fish samples from affected and unaffected areaFish Arsenic Content(µg kg-1)

Mean ±SE Min Max 25%50%Median

75% 95%

Contaminated Villages Sample no=295

100 6 BDL 1000 39 68 131 280

Uncontaminated Village Sample no= 92

19 4 BDL 138 0 0 28 109

In the contaminated zone, Catla catla exhibited maximum accumulation (60% samples >100 µg kg-1) followed by Labeo rohita (39% samples >100 µg kg-1), Cirrhinus mrigala (24% samples >100 µg kg-1) and Labeo bata (18% samples >100 µg kg-1). Arsenic was also noticed in the fleshes of Channa sp (34% samples >100 µg kg-1). A positive correlation was observed between arsenic content in fish flesh and that of pond water (pearsons correlation coefficient 0.388, n= 294) and sediment (pearsons correlation coefficient 0.462, n= 294) of the pond from where it has been collected. Both correlations were significant at 0.01% level. Bivalve and gastropod samples from affected zone were found with As contamination in the range of 260 - 3068 µg kg -1 (average 990 ± 140 µg kg-1) and those from uncontaminated zone in the range of 115 - 1556 µg kg-1 with an average value 701 ± 214 µg kg-1.

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Conclusion

Both the review and the present case study emphasized that Arsenic contaminations in the abiotic components of inland ecosystems are not very alarming unless any point source of contamination and the beneficial role of using these ecosystems over the highly contaminated ground water for various livelihood activities. The case study revealed that both the water and sediment phases of affected areas were found quite contaminated with respect to the ponds from relatively unaffected area of Hooghly district from the Gangetic delta region of West Bengal. Since the average fish consumption of the resident population is low compared to the contaminated drinking water and rice, the exposure from fishes is relatively less. In fact, fishes will help more in combating arsenic toxicity. It is however better to avoid gastropods and bivalves in the diets since in these species arsenic accumulation is found to be high.

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